The present invention relates to a polarizing plate and liquid crystal display, particularly to a polarizing plate and liquid crystal display wherein the fluctuation of tint (tincture) due to humidity change is kept suppressed when the liquid crystal display of VA mode is observed from off-center or from the direction of slant.
The cellulose ester represented by the triacetyl cellulose employed as a polarizing plate protective film is a hydrophilic polymer, as compared to polycarbonate or norbornane based polymer. When the polymer is hydrophilic, the adhesive surface in contact with the polarizing film mainly made up of polyvinyl alcohol (PVA) is characterized by extremely easy handling. Conversely, being hydrophilic (hygroscopic) involves a problem of fluctuation of the tint in a slanting direction resulting from environmental changes such as temperature and humidity. To suppress such environmental changes, countermeasures have been taken in the conventional art by changing the type of plasticizer, increasing the amount thereof or introducing a rod-like compound for the purpose of reducing the percentage of water absorption (e.g. Patent Documents 1 and 2).
The fluctuation of the tint in a slanting direction can be improved to some extent by reducing the percentage of water absorption, for example, through increase in the amount of plasticizer. However, a problem has arisen in the practical use when compared to the case of using the less hydrophilic material such as polycarbonate and norbornane based polymer (e.g. Patent Document 3).
The present inventors have found out that what is important to suppress the fluctuation of tint in a slanting direction is to suppress the fluctuation in the front contrast (amount of white light/amount of black light) and the fluctuation in the leakage of front light when black is displayed, and the aforementioned reduction in the percentage of water absorption is less important. To achieve this, it is important to taken into account not only reduction in the percentage of water absorption by the plasticizer and others, but also the production process conditions and interaction by use of other additives.
Normally, in addition to the plasticizer, the following substances may be added to the polarizing plate protective film: polyvinyl alcohol (PVA); ultraviolet absorber for protecting the dye and liquid crystal cell including the PVA and iodine against ultraviolet rays; and particles for providing sliding property in order to maintain the film winding form. To improve the quality of the polarizing plate, these additives are essential.
In the present invention, it has been found out that fluctuation in tint can be suppressed if the environmental fluctuation in the leakage of front light during display in black is kept within the range determined in accordance with the present invention. However, reduction of the environmental fluctuation in the leakage of front light during display in black requires the fluctuation of depolarization to be suppressed. Even if the phase difference film using a hydrophilic polymer is employed in the polarizing plate protective film mere control of the incoming and outgoing water into and out of the film is not effective for the fluctuation of depolarization. Presence of an ultraviolet absorber or particle for incoming and outgoing water will cause fluctuation in the degree of depolarization. This will cause environmental fluctuation of tint when observed in a slanting direction. However, complete elimination of all of these additives is not preferred from the viewpoint of the liquid crystal display.
The present inventors have made efforts to solve the aforementioned problem, and have found out that the environmental fluctuation in the leakage of front light during the display of black can be suppressed by controlling the production process conditions and the amounts of the ultraviolet absorber and particle
[Patent Document 1] Japanese Non-Examined Patent Publication 2002-267847
[Patent Document 2] Japanese Non-Examined Patent Publication 2004-4550
[Patent Document 3] Japanese Non-Examined Patent Publication 2005-114995